The present invention relates to perpendicular magnetic recording medium which are small in read back noise and suitable for high-density magnetic recording, and to a magnetic recording apparatus using these media.
The currently used practical magnetic recording system is the longitudinal magnetic recording system in which magnetic recording is made in parallel to the surface of the magnetic recording medium, and so that the magnetic N-poles are opposed one to one and that the magnetic S-poles are opposed to one to one. In order to increase the linear recording density in the longitudinal magnetic recording, it is necessary that the coercivity be increased by decreasing the product of residual flux density (Br) and magnetic film thickness (t) of the magnetic film of a recording medium to reduce the effect of the demagnetizing field at the magnetic recording process. In addition, to decrease the medium noise caused by magnetization transition, it is necessary to orient the magnetic easy axis of the magnetic film in the direction parallel to the substrate surface, and to control the crystal grain size. To control the crystal orientation and grain size of the magnetic thin film, an underlayer for structure control is formed between the substrate and the magnetic film.
As the magnetic film, a Co-based alloy thin film is used which chiefly contains cobalt Co, and has an element such as Cr, Ta, Pt, Rh,.Pd, Ti, Ni, Nb, Hf added to the cobalt. The Co-based alloy of the magnetic thin film is chiefly made of a material of hexagonal closed packed structure (hereinafter, referred to as hcp structure). The magnetic easy axis is present in the directions of c-axis,  less than 0, 0.1 greater than  and is oriented in the longitudinal direction. An underlayer for structure control is formed between the substrate and the magnetic film in order to control the crystal orientation and grain size of the magnetic film. The underlayer is made of a material which chiefly contains Cr and has an element of Ti, Mo, V, W, Pt, Pd added to Cr. The magnetic thin film is formed by vacuum evaporation or sputtering. As described above, the product of residual flux density (Br) and magnetic film thickness (t) of the magnetic film is required to be reduced in order to reduce the medium noise in the longitudinal magnetic recording and to thereby increase the linear recording density. For this purpose, it has been considered that the magnetic film thickness is reduced to 20 nm or below, and that the crystal grain size is greatly reduced. However, such medium has a very important problem that the recording magnetization is reduced by thermal instability. This phenomenon interferes with the high density recording.
The presently used magnetic disk recording apparatus is of the longitudinal magnetic recording system. The technical subject is that the longitudinal domains parallel to the substrate are formed at high density in the longitudinal magnetic recording medium which is easy to be magnetized in the direction parallel to the disk substrate surface. A method of increasing the recording density in the longitudinal magnetic recording medium is proposed, which employs keepered media formed by depositing an extremely thin soft magnetic film on the recording medium that have the magnetic easy axis in the longitudinal direction.
This technique is described in Abstracts, page 116 (paper No. DQ-13) and pages 133-134 (paper No. EB12) published in the 41st Annual Conference on Magnetism and Magnetic Materials (Nov. 12-15, 1996).
In such documents, it is described that if this media structure is employed it will be possible to increase the longitudinal magnetic recording density to 1 Gb/in2 or above by use of thin film heads of self-recording/reproduction system. In the longitudinal recording system, however, since the adjacent recorded bits are substantially magnetized to oppose to each other, magnetization transition regions of certain widths are formed across the boundaries even when this technique is employed. Thus, it will be technically difficult to record at a longitudinal density of 5 Gb/in2 or above.
On the other hand, the perpendicular magnetic recording system forms domains on the recording medium surface perpendicularly, and so that the adjacent recorded bits are unti-parallel to each other. This system has the advantage that the demagnetizing field at the boundary between the recorded bits is decreased, and thus it is one of the powerful means for high-density magnetic recording.
For the longitudinal high-density recording, it is necessary that the magnetic film be formed to have a thickness of 20 nm or below as described above. In this case, there is a problem that the recorded magnetization regions may be lost by thermal instability. On the contrary, the perpendicular magnetic recording system is able to form the magnetic film thicker than the longitudinal magnetic recording system, and thus stably maintain the recorded magnetization regions. In order to reduce the media noise generated from the magnetization transition and increase the linear recording density in the perpendicular recording system, it is necessary to orient the magnetic easy axis of the magnetic film perpendicularly to the substrate surface, and control the crystal grain size.
As the magnetic film, a Co-based alloy thin film is used which chiefly contains cobalt Co and has an element of Cr, Ta, Pt, Rh, Pd, Ti, Ni, Nb, Hf added to Co. The Co-based alloy of the magnetic thin film is chiefly made of a material of hexagonal closed packed structure (hereinafter, referred to as hcp structure). The magnetic easy axis is present in the directions of c-axis,  less than 0, 0.1 greater than  and is oriented in the perpendicular direction. The magnetic thin film is formed by vacuum evaporation or sputtering. In order to increase the linear recording density at the time of recording and the read output and reduce the read back noise so that the magnetic recording characteristics can be improved, it is necessary to improve the perpendicular orientation of the c-axis of the Co-based alloy thin film and control the crystal grain size. Thus, it has so far been considered that an underlayer for structure control is formed between the substrate and the magnetic film.
The perpendicular magnetic recording system has attracted a great deal of attention as a system capable of high-density magnetic recording, and structure of medium suitable for the perpendicular magnetic recording have been proposed. A method of providing a non-magnetic underlayer between the perpendicular magnetization film and the substrate has been examined in order to improve the perpendicular orientation of the perpendicularly magnetized film made of a Co-based alloy material. For example, a method of depositing a Ti film as an underlayer for a Coxe2x80x94Cr magnetic film is described in JP-A-5877025, and JP-A-58-141435, a method of providing an under layer made of Ge, Si material in JP-A-60-214417, and a method of providing an underlayer made of an oxide material such as CoO, NiO in JP-A-60-064413. In addition, a magnetic recording medium having a soft magnetic layer of Parmalloy provided between the substrate and the perpendicular magnetization film has been considered as a perpendicular magnetic recording medium which is used in combination with a single-pole type magnetic recording head.
However, in order to achieve ultra-high density magnetic recording of several Gb/in2 or above, particularly more than 10 Gb/in2, it is important to reduce the noise contained in the read output signal, particularly the medium noise caused by the micro-structure of the medium in addition to the improvement of the linear recording density. Thus, it is necessary to more highly control the thin film structure in addition to the crystal orientation of the magnetic film. The media noise has so far been tried to reduce by various ways. For example, these ways are (1) to segregate the nonmagnetic element Cr of CoCr-based alloy in the crystal grain boundary or grains in order to suppress the magnetic mutual action between the magnetic grains and (2) to isolate the magnetic grains in a form by controlling the sputtering gas pressure. The improvement in medium structure by these conventional techniques was effective in reducing the medium noise, but the reverse magnetic domains formed opposite to the magnetization direction and the associated magnetization irregularity, which cause the medium noise in the perpendicular magnetic recording, was not able to be reduced effectively.
The perpendicular magnetic recording medium capable of high-density magnetic recording of 5 Gb/in2 or above is required to have small medium noise in addition to high linear recording density or resolution. There are some reported examples. As for example described in a paper titled xe2x80x9cImprovement in S/N ratio of Single Layer Perpendicular Magnetic Disk Mediaxe2x80x9d of the Fifth Perpendicular Magnetic Recording Symposium (Oct. 23-25, 1996), pp. 98-103, it is effective to decrease the thickness of the perpendicular magnetization film, introduce a non-magnetic underlayer of CoCr between the perpendicular magnetization film and the substrate, add a non-magnetic element such as Ta to the Co alloy magnetic film and/or reduce the magnetic crystal grain size. These countermeasures are able to considerably reduce the medium noise, but if the noise can be more decreased, the recording density of the magnetic recording apparatus will be easily increased much more.
Accordingly, it is an object of the invention to remove the drawbacks of the prior art, and provide perpendicular magnetic recording media having excellent-low-noise characteristics and suitable for ultra-high density magnetic recording by controlling the perpendicular magnetic anisotropy, crystal orientation or mutual action between the magnetic grains of the perpendicular magnetization film formed on the substrate to thereby control the fine domain structure magnetically recorded, and a magnetic recording apparatus using the medium.
According to the invention, the perpendicular magnetic recording medium having the low-noise characteristics can achieve high-density recording of 5 Gb/in2 or above, and thus make it easy to produce high-density recording apparatus.
After examining the recorded magnetization structure of the perpendicular magnetic recording medium by a magnetic force microscope and a spin-polarized scanning electron microscope, it was found that most of the noise are caused by reverse magnetic domains and microscopic instability of magnetization present in the medium surfaces. In order to decrease the medium noise, it is necessary to reduce the reverse magnetization and microscopic instability of magnetization present in the medium surfaces.
It is an object of the invention to provide perpendicular magnetic recording medium having both low noise characteristics and high-density linear recording characteristic which enable the high-density magnetic-recording of 5 Gb/in2 or above, and magnetic recording/reproducing apparatus using the medium.
According to the invention, at least two magnetic films which are different in perpendicular magnetic anisotropy are formed on the substrate, and in this case the perpendicular magnetic anisotropy of the second magnetic film on the side far from the substrate is made greater than that of the first magnetic film on the side near to the substrate surface, thereby improving the magnetic isolation of the first magnetic film on the side near to the substrate surface. That is, the above object can be achieved by assigning a different role to each magnetic film as they say.
After examining the recorded magnetization structure by a magnetic force microscope or spin polarized scanning electron microscope, it was found that most of the noise are caused by the reverse magnetic domains and microscopic instability of magnetization present in the medium surfaces. In order to decrease the medium noise, it is necessary to reduce the reverse magnetic domains and microscopic instability of magnetization present in the medium surfaces. To decrease the medium noise and assure high-density linear recording characteristics, it is desired to reduce the magnetic crystal grain size of the perpendicular magnetic recording medium and magnetically isolate the grains.
From the results of experiments by the inventors, it was found that the following media structure is effective in achieving the above object.
First, referring to FIG. 1, a single-layer type perpendicular magnetic recording medium is normally produced by depositing a perpendicular magnetization film 13 on an underlayer 12 which is formed on a non-magnetic substrate 11 for the purpose of improving the perpendicular orientation of the magnetic film and controlling the crystal grain size. On this magnetization film, there is usually deposited a protective film of carbon or the like. The magnetic film is made of a Co-based alloy containing at least one element selected from Cr, Ta, Pt, Pd, Si, V, Nb, W, Mo, Hf, Re, Zr, B, P, Ru. This magnetic film is a polycrystalline film, and for high density linear recording characteristic and low-noise characteristic, its crystal grain size is selected to be normally 20 nm or below, and a non-magnetic element is preferentially segregated in the crystal grain boundary. This perpendicular magnetization film has a small magnetic exchange coupling force in the longitudinal direction because the segregated layer exists in the crystal grain boundary.
The present inventors found that the effective way to further reduce the medium noise is to provide on the first perpendicular magnetization film 13 of Co-based alloy a second perpendicular magnetization film 14 of which the magnetic exchange coupling force in the longitudinal direction is greater than that of the first perpendicular magnetization film. The second perpendicular magnetization film 14 is preferably a multi-layered perpendicular magnetization film of Co/Pt, Co/Pd, Co alloy/Pt, Co alloy/Pd, Co alloy/Pt or Pd alloy or a noncrystal perpendicular magnetization film of Tb Fe Co containing rare-earth elements.
The deposition of the second perpendicular magnetization film 14 can reduce the magnetic instability present in the surface of the first perpendicular magnetization film 13. Since the second perpendicular magnetization film 14 has a large magnetic exchange coupling force in the longitudinal direction, the microscopic magnetic instability is not easily caused on the surface.
Here, in order to assure low-noise characteristics in the perpendicular magnetic recording medium, it is necessary to decrease the thickness of the second perpendicular magnetization film 14 as compared with that of the first perpendicular magnetization film 13. The thickness of the second perpendicular magnetization film is preferably selected to be less than ⅓ that of the first perpendicular magnetization film. The role of the second perpendicular magnetization film 14 is not to hold the perpendicular magnetic recording, but to greatly reduce the microscopic magnetic instability in the surface of the first perpendicular magnetization film 13. To assign this function to a thin film, it is more desirable to provide high magnetic anisotropy energy of 5xc3x97106 erg/cc or above. When the thickness of the second perpendicular magnetization film 14 is larger than that of the first perpendicular magnetization film 13, the medium noise is increased as compared with the case in which only the first perpendicular magnetization film is deposited.
In order to achieve a recording density of 5 Gb/in2 or above, the total thickness of the first and second perpendicular magnetization films should be selected to be more than 7 nm and less than 100 nm. If the total thickness is larger than 100 nm, the magnetic crystal grains constituting the perpendicular magnetization films are enlarged in volume, and as a result the magnetic switching volume is also increased, thus causing large medium noise. Accordingly, it is not possible to achieve the signal to noise ratio for a recording density of 5 Gb/in2 or above. If the total thickness is equal to or less than 7 nm, the recording magnetization is remarkable deteriorated by thermal instability.
The thickness of the second perpendicular magnetization film is preferably more than 3 nm and less than 10 nm. If it is equal to or less than 3 nm, the effect of reducing the magnetic instability in the surface of the first perpendicular magnetization film is unrecognizably small. If it is larger than 10 nm, the medium noise is increased.
The object of the invention can also be achieved by employing the structures shown in FIGS. 2 through 6. As to the structure shown in FIG. 2, the second perpendicular magnetization film 14 is deposited on the first perpendicular magnetization film 13 as in the case of FIG. 1, but between the first perpendicular magnetization film 13 and the non-magnetic substrate 11 there are formed an underlayer 23 of a nonmagnetic material having a hexagonal closed packed structure or a weak ferro magnetic material having a hexagonal closed packed structure of which the saturation magnetization is 100 emu/cc or below, and the underlayer 12 for controlling the crystal orientation of this underlayer film. Use of these dual-underlayer structure will enable the first perpendicular magnetization film to be highly controlled in its crystal grain size and orientation, and low-noise characteristics to be achieved.
The media structure, as shown by its cross section in FIG. 3, has the first and second perpendicular magnetization films 13, 35 stacked, and also has another perpendicular magnetization film 33 of multi-layered structure or non-crystal structure formed between the first perpendicular magnetization film 13 and the underlayer 12. Thus, the magnetic instability present in the front and back sides of the first perpendicular magnetization film 13 can be reduced, so that low-noise characteristics can be achieved.
The cross-sectional structures shown in FIGS. 4 through 6 are structures of perpendicular magnetic recording media of the type in which a soft magnetic layer is provided under the perpendicular magnetization film as indicated by reference numeral 42. In this case, the second perpendicular magnetization film 44 of multi-layered structure or amorphous-like structure is also provided on the first perpendicular magnetization film 13 of Co alloy.
Referring to FIG. 5, there is shown an underlayer film 53 having a non-magnetic hexagonal closed packed structure or a weak ferro magnetic hexagonal closed packed structure of which the saturation magnetization is 100 emu/cc or less. In FIG. 6, there is shown a perpendicular magnetization film 64 of multi-layered or amorphous structure.
The second perpendicular magnetization film of multi-layered or amorphous structure is formed on the first perpendicular magnetization film as shown in FIGS. 1 through 6. This second perpendicular magnetization film serves to reduce the microscopic magnetic instability present in the surface of the first perpendicular magnetization film. The perpendicular magnetization film of multi-layered or non-crystal structure formed under the first perpendicular magnetization film as shown in FIGS. 3 and 6 acts to reduce the microscopic magnetic instability present in the lower surface of the first perpendicular magnetization film.
The underlayer 12 in FIG. 1, the underlayer film 23, shown in FIG. 2, of non-magnetic hexagonal closed packed structure or weak ferro magnetic hexagonal closed packed structure of which the saturation magnetization is 100 emu/cc or below, the underlayer 12 formed thereunder, the underlayer shown in FIG. 3, the under layer 53, shown in FIG. 5, of non-magnetic hexagonal closed packed structure or weak ferro magnetic hexagonal closed packed structure of which the saturation magnetization is 100 emu/cc or below, and the underlayer 12 in FIG. 6 are all provided for the purpose of controlling the crystal orientation and crystal grain size of the magnetic films formed on these underlayers, respectively, thus making it possible to improve the characteristics of the magnetic films along their purposes. If the saturation magnetization of the underlayer film exceeds 100 emu/cc, the medium noise will be increased, and the recording resolution will be reduced, thus adversely affecting the magnetic recording/reproduction characteristics.
As to the cause of the reverse magnetic domain, if the perpendicular magnetization film is perpendicularly magnetized in one direction, an intense demagnetizing field is acted on the medium surface, and this action of demagnetizing field produces the so-called reverse magnetic domains in the direction opposite to the perpendicularly magnetized direction. In order to suppress this reverse magnetic domains from being produced, it is necessary to employ a perpendicular magnetization film having magnetic anisotropy energy. The magnetic anisotropy energy is desirably 2.5xc3x97106 erg/cc or above.
The magnetic anisotropy energy of the perpendicular magnetization film made of Co-based alloy easy to handle as practical media is 5xc3x97106 erg/cc. There is a Co-based alloy regular lattice material having more magnetic anisotropy energy than this value, but this material has difficulty in reducing noise because too large media noise is caused by a strong mutual action in the longitudinal direction of the magnetic film.
The multi-layered perpendicular magnetization film of Pt/Co, Pd/Co other than Co alloy or a perpendicular magnetization film of amorphous structure containing rare earth elements such as TbFeCo has magnetic-anisotropy energy of 2.5xc3x97106 erg/cc or above and thus can be expected to attain the present subject. However, the magnetic mutual action in the longitudinal direction will also be strong as in the above description if no countermeasure is provided, and hence the medium noise is large. Thus, a special device is needed for reducing the media noise. In addition, in order to increase the surface magnetic recording density up to 5 Gb/in2 or above, the linear recording density is required to be 250 kFCI or above. The bit length corresponding to this linear recording density is 100 nm. The thickness of the magnetic recording media for recording is desired to be less than the minimum bit length. Thus, the thickness of the perpendicular magnetization film is required to be 100 nm or below.
Since the reverse magnetic domains can be suppressed by use of a perpendicular magnetization film of high magnetic anisotropy energy, the medium noise due to the reverse magnetic domains can be prevented from being produced. However, the medium noise is also produced by a microscopic magnetic instability present in the medium surfaces. If the magnetic mutual action in the longitudinal direction of the magnetic film is large, a long-period magnetizing instability is produced. In addition, it was found that if there is magnetic heterogeneity in the surface of the perpendicular magnetization film, a short-period magnetizing instability is produced, thus causing the medium noise.
From the results of the experiments by the inventors, it was found that in order to suppress these long-period, short-period magnetic instability, it is necessary that a soft magnetic film or a magnetic film having a magnetic easy axis in the longitudinal direction be formed on the surface of the perpendicular magnetization film. In this case, the thickness of the magnetic films must be selected to be smaller than that of the perpendicular magnetization film for recording. If the film thickness is large, the magnetic flux produced from the recorded bits on the perpendicular magnetization films makes closed magnetic circuits absorbed by these films, and thus the magnetic flux does not leak from the medium surfaces, so that the recorded signal cannot be reproduced by magnetic heads.
If the thickness of these magnetic films is properly thin, the long-period, short-period magnetic instability present in the perpendicular magnetization film surfaces is absorbed by the magnetic films formed on the surfaces of the perpendicular magnetization films, but the strong magnetic flux generated from the recorded bits cannot be absorbed enough. In this case, since the magnetic flux caused by the magnetic instability does not leak to the magnetic film surfaces, it cannot be detected by magnetic heads, so that the medium noise is decreased. The combination of the perpendicular magnetization film which absorbs the magnetic flux due to the magnetic instability but little absorbs the magnetic flux from the recorded bits, and the thickness of the magnetic film provided on the surface depends on the thickness of the perpendicular magnetization film, saturation magnetization, coercivity and so on. Thus, it is necessary to select the most suitable combination for each case.
The thickness of the magnetic film provided on the surface should be confined within a range of generally 2 to 10 nm or preferably 3 to 5 nm considering the factors such as controllable ability for film production and for film thickness distribution in all regions of disk surfaces. The magnetic film may be a soft magnetic film of Parmalloy, Fexe2x80x94Si, Fexe2x80x94Sixe2x80x94Al, CoNbZr or other materials or a magnetic film, of Co, Ni, Fe, CoNi, CoNiCr or the like, which is easy to be magnetized in the longitudinal direction. In addition, part of the surface of the perpendicular magnetization film may be changed to a soft magnetic film or longitudinal magnetization film by diffusing or implanting a light element such as C, B, N or P into the surface.
FIG. 7 shows an example of the application of the invention to a single-layered perpendicular magnetic recording medium.
In this single-layered perpendicular magnetic recording medium, normally the underlayer 12 for improving the perpendicular orientation of the magnetic film and controlling the crystal grain size is deposited on the non-magnetic substrate 11, and a perpendicular magnetization film 71 is formed on the underlayer. In addition, a protective film of carbon or the like is normally deposited on the magnetization film.
The magnetic film 71 is made of a Co-based alloy containing at least one of Cr, Ta, Pt, Pd, Si, V, Nb, W, Mo, Hf, Re, Zr, B, P, Ru. This magnetic film is a poly-crystal film of which the crystal grain size is normally 20 nm or below and which has the structure in which a non-magnetic element is preferentially segregated in the crystal grain boundaries in order to attain high density linear recording characteristics and low noise characteristic. This perpendicular magnetization film has a small magnetic exchange coupling force because of the presence of the segregated layer in the crystal grain boundaries, and a microscopic magnetic instability because of the compositional segregation and undulations in the media surfaces.
To reduce the medium noise, a soft magnetic film or longitudinal magnetization film 72 which is thinner than the thickness of the perpendicular magnetization film is deposited on the perpendicular magnetization film 71. The perpendicular magnetization film 71 may be a multi-layer type perpendicular magnetization film made of Co/Pt, Co/Pd, Co alloy/Pt, Co alloy/Pd, Co alloy/Pt or Pd alloy, or a amorphous type perpendicular magnetization film containing a rare earth element such as TbFeCo in place of being made of a Co-based alloy material.
When the soft magnetic film 72 is provided on the perpendicular magnetization film 71, the magnetic flux generated from the magnetic instability present in the surface of the perpendicular magnetization film 71 is absorbed by the magnetic film, and as a result the medium noise is decreased. In addition, when a thin magnetic film is formed, the magnetic instability is not easily generated even if microscopic undulations and composition segregation are present in the surface. As shown in FIG. 7, the protective film 15 is deposited on the magnetization film.
In order to achieve a recording density of 5 Gb/in2 or above, the total thickness of the perpendicular magnetization film 71 and soft magnetic film 72 is required to be confined within the range of 7 nm to 100 nm. If the thickness is larger than 100 nm, the volume of magnetic crystal grain constituting the film becomes large, and as a result the magnetic switching volume is enlarged. Thus, the medium noise is increased, degrading the signal to noise ratio, so that a recording density of 5 Gb/in2 or above cannot be attained. If the thickness is smaller than 7 nm, the recording magnetization is remarkably deteriorated by thermal instability.
The object can also be achieved by use of the structures shown in FIGS. 8 through 10. In the medium structure shown in FIG. 8, an underlayer 73 having a nonmagnetic hexagonal closed packed structure or a weak ferro hexagonal closed packed structure of 100 emu/cc or below is formed between the first perpendicular magnetization film 71 and the non-magnetic substrate 11, and the underlayer 12 is also formed under the underlayer 73 in order to control the crystal grain orientation of the underlayer film 73. In addition, the magnetic film 72 made of a thin soft magnetic film or longitudinal magnetization film is deposited on the perpendicular magnetization film 71. By this dual underlayer structure, it is possible to highly control the crystal grain size and crystal orientation of the perpendicular magnetization film, and hence to attain low noise characteristics.
The structures shown by the cross-sectional views of medium of FIGS. 9 and 10 are of the perpendicular magnetic recording medium of the type in which a soft-magnetic film layer is held under the perpendicular magnetization film. As shown in FIGS. 9 and 10, a soft magnetic film layer 74 is formed on the substrate. In this case, the magnetic film 72 of a soft magnetic film or having longitudinal magnetic anisotropy is also formed on the perpendicular magnetization film.
In FIGS. 7 through 10, the magnetic film formed on the perpendicular magnetization film absorbs the magnetic flux generated from the long-period, short period magnetic instability present in the surface of the perpendicular magnetization film, thus serving to reduce the medium noise. The underlayer 12 in FIG. 7, the underlayer 73 having a non-magnetic hexagonal closed packed structure or a weak ferro hexagonal closed packed structure of which the saturation magnetization is 100 emu/cc or below, and the underlayer 12 under it as shown in FIG. 8, and the underlayer 73 in FIG. 10 are all provided for the purpose of controlling the crystal orientation and crystal grain size of the magnetic film formed on the underlayers. Thus, the magnetic film can be improved in the characteristics.
In addition, the protective film 15 is formed as shown in the figures.